1. Field of the Invention
The present invention relates to imaging apparatuses and photoconductors, and especially to an imaging apparatus which performs development almost simultaneously with imaging light exposure of a photoconductor from the inside thereof to obtain a toner image on the photoconductor, for great improvement over the conventional Carlson process, with no generation of ozone which is harmful to humans, and which consistently provides satisfactory images at low cost. With the rapid developments in computer and communication technology in recent years, the demand for printers as output terminals has been increasing. Electrophotographic printers are rapidly becoming commonplace because of their excellent recording speed and print quality. The present invention is directed to the development of such printers, digital copiers and fax machines.
2. Description of the Related Art
In the conventional electrophotographic process (Carlson process), a photoconductor is used as a recording medium and the recording is carried out by a complicated series of steps including electrification, light exposure, development, transfer, fixation, destaticizing and cleaning, which have limited the miniature, low-cost and maintenance-free aspects of the devices, and created the desire for a more simple developing process. Recently, attempts have been made at development using transparent photoconductors, and it has been reported that by eliminating the electrifying mechanism of the above-mentioned conventional process and also situating the optical system inside the photoconductor, further miniaturization is possible. In Japanese Unexamined Patent Publication (Kokai) No. 6-273964 for example, an organic photoconductor is used for development with magnetic toner and a high resistance carrier.
This principle will now be explained.
The basic principle of an imaging apparatus employing the process described above is shown in FIG. 1 and FIGS. 2A to 2C. The photoconductor 1 comprises a transparent substrate 2, a transparent conductive layer 3 and a photoconductive layer 4, and the transparent conductive layer is grounded. The developing agent 5 used contains a high-resistance carrier 6 and insulating toner 7. A developing roller 8 is provided with a conductive sleeve 10 on a magnet roller 9, and the developing agent is pulled in the direction of the developing roller by magnetic force, and adheres to the sleeve while being carried to the photoconductor 1. Also, three successive steps are carried out almost instantaneously in the developing nip. First, in zone (1), the photoconductor 1 is electrified 12 by the developing agent 5. Next, in zone (2), imaging light exposure is performed on the electrified photoconductor 1 from the transparent substrate 2 side, to form a latent image. The number 11 indicates an optical system. Also, in zone (3), development occurs in the latent image-formed areas because the electrical adhesive force 13 of the toner 7 on the photoconductor 1 is stronger than the magnetic force 14 from the magnet roller 9, and conversely, in the background areas other than the image-formed areas the toner 7 is collected because the magnetic electrostatic force from the magnet roller 9 is stronger. The developed toner 7 is transferred to the recording medium, i.e. the paper or plastic plate, to obtain a print. Here, the direction of rotation of the photoconductor drum and the developing agent sleeve may be in the same or different directions. The image recording process described above will hereunder be referred to as "rear photorecording process".
The differences between this rear photorecording process and the Carlson process will now be discussed. FIG. 3 shows an apparatus used for the Carlson process, and FIG. 4 shows an apparatus used for the rear photorecording process.
In FIGS. 3 and 4, 21 is a photoconductor drum (non-transparent), 22 is an electrifier, 23 is the surface potential, 24 is an optical system, 25 is a developer, 25a is a developing agent, 26 is toner, 27 is a recording sheet, 28 is a transfer unit, 29 is a fixing unit, 30 is a destaticizing lamp, 31 is a cleaner, 32 is a photoconductor drum (transparent support) and 33 is a transfer roller.
As is well-known, in the Carlson process the electrification, exposure and development of the photoconductor are usually carried out in separate processing zones, and therefore the electrification potential (absolute value) of the photoconductor may be set higher than the developing bias, so that no fog occurs. That is, in the conventional process as shown in FIGS. 5 and 6, the toner is carried electrostatically to the latent image, but the toner does not adhere to the background sections because of electrical repulsion. However, in the rear photorecording process, it is believed that a surface potential is generated on the photoconductor by the charge injection and microdischarge due to the developing bias (V.sub.b) upstream from the photoconductor in the developing nip; nevertheless, since the efficiency is low when using a common photoconductor, the potential of the photoconductor is lower than the developing bias. The difference between the developing bias and the surface potential of the photoconductor is more apparent the higher the toner concentration (FIG. 7). Consequently, when magnetic toner is used, lower toner concentrations (7 wt % or less) make the surface potential of the photoconductor closer to the developing bias and thus reducing fog, while higher toner concentrations (10 wt % or greater) lower the surface potential of the photoconductor and render it prone to fog. Thus, when the surface potential (V.sub.s) becomes lower than the developing bias (V.sub.b) due to the toner concentration, a developer construction which does not allow control of the toner concentration (such as in Japanese Unexamined Patent Publication No. 5-150667) cannot be used. Also, when a conventional two-component developer is used which employs a magnetic permeability sensor to control the toner concentration, since both the toner and carrier are magnetic, strict control is difficult even in the case of low toner concentrations, while lot differences tend to occur with the photoconductor, etc., making it thus difficult to achieve a satisfactory margin against fog.
In addition, since in the case of non-magnetic toner such as normal color toner, there is no dependence on the toner concentration and the magnetic collecting force of the toner does not apply, the surface potential (V.sub.s) cannot be higher than the developing bias (V.sub.b), and fog has resulted.
Consequently, with magnetic toner the surface potential (V.sub.s) is either made to approach the developing bias (V.sub.b) or is made higher than the developing bias (V.sub.b), to provide satisfactory printing characteristics in a wide range of toner concentrations, and to increase the anti-fog margin. Furthermore, if the surface potential of the photoconductor can be made larger than the developing bias in the case of non-magnetic color toner as well, developing may be made without fog.